1. Field of the Invention
The present invention relates to an organometallic complex. In particular, the present invention relates to an organometallic complex that is capable of converting a triplet excited state into luminescence. In addition, the present invention relates to a light emitting element, a light emitting device and an electronic device which use the organometallic complex.
2. Description of the Related Art
Organic compounds are brought into an excited state by absorbing light. Through this excited state, various reactions (such as photochemical reactions) are caused in some cases, or luminescence is generated in some cases. Therefore, various applications of the organic compounds have been made.
As one example of the photochemical reactions, a reaction (oxygen addition) of singlet oxygen with an unsaturated organic molecule is known (refer to Reference 1: Haruo INOUE, et al., Basic Chemistry Course PHOTOCHEMISTRY I (Maruzen Co., Ltd.), pp. 106-110, for example). Since the ground state of an oxygen molecule is a triplet state, oxygen in a singlet state (singlet oxygen) is not generated by a direct photoexcitation. However, in the presence of another triplet excited molecule, singlet oxygen is generated, which can lead to an oxygen addition reaction. In this case, a compound that can become the triplet excited molecule is referred to as a photo sensitizer.
As described above, in order to generate singlet oxygen, a photosensitizer that can become a triplet excited molecule by photoexcitation is necessary. However, since the ground state of an ordinary organic compound is a singlet state, photoexcitation to a triplet excited state is a forbidden transition, and a triplet excited molecule is not easily generated. Therefore, as such a photosensitizer, a compound in which intersystem crossing from the singlet excited state to the triplet excited state easily occurs (or a compound which allows the forbidden transition of photoexcitation directly to the triplet excited state) is required. In other words, such a compound can be used as a photosensitizer and is useful.
Also, such a compound often emits phosphorescence. The phosphorescence is luminescence generated by transition between different energies in multiplicity and, in the case of an ordinary organic compound, the phosphorescence indicates luminescence generated in returning from the triplet excited state to the singlet ground state (in contrast, luminescence in returning from a singlet excited state to the singlet ground state is referred to as fluorescence). Application fields of a compound capable of emitting phosphorescence, that is, a compound capable of converting a triplet excited state into luminescence (hereinafter, referred to as a phosphorescent compound), include a light emitting element using an organic compound as a light emitting substance.
This light emitting element has a simple structure in which a light emitting layer including an organic compound that is a light emitting substance is provided between electrodes. This light emitting element is a device attracting attention as a next-generation flat panel display element in terms of characteristics such as being thin and light in weight, high speed response, and direct current low voltage driving. In addition, a display device using this light emitting element is superior in contrast, image quality, and wide viewing angle.
The emission mechanism of a light emitting element in which an organic compound is used as a light emitting substance is a carrier injection type. Namely, by applying voltage with a light emitting layer interposed between electrodes, electrons and holes injected from the electrodes are recombined to make the light emitting substance excited, and light is emitted in returning from the excited state to the ground state. As the type of the excited state, as in the case of photoexcitation described above, a singlet excited state (S*) and a triplet excited state (T*) are possible. Further, the statistical generation ratio thereof in a light emitting element is considered to be S*:T*=1:3.
As for a compound capable of converting a singlet excited state to luminescence (hereinafter, referred to as a fluorescent compound), luminescence from a triplet excited state (phosphorescence) is not observed but only luminescence from a singlet excited state (fluorescence) is observed at a room temperature. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) in a light emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on S*:T*=1:3.
On the other hand, when the phosphorescent compound described above is used, the internal quantum efficiency can be improved to 75 to 100% in theory. Namely, a light emission efficiency that is 3 to 4 times as much as that of the fluorescence compound can be achieved. For these reasons, in order to achieve a highly-efficient light emitting element, a light emitting element using a phosphorescent compound has been developed actively (for example, refer to Reference 2: Zhang, Guo-Lin, et al., Gaodeng Xuexiao Huaxue Xuebao (2004), vol. 25, No. 3, pp. 397-400). In particular, as the phosphorescent compound, an organometallic complex using iridium or the like as a central metal has been attracting attention, owing to its high phosphorescence quantum yield.